The present invention relates in general to inflators for use in airbag restraint systems. In particular, the present invention relates to an improved hybrid inflator having reduced particulates in the generated gas.
Airbag restraint systems for vehicles typically include a collision sensor, a gas generator or inflator operatively connected to the collision sensor, and a cushion which receives the gas generated by the inflator. When the collision sensor detects a collision, it sends a signal which activates the inflator. The inflator then produces or releases a quantity of inflation gas into the cushion. The cushion is inflated by this gas, providing a restraint for the vehicle occupant.
Various types of inflators are known, and are generally grouped according to their operating principles. For example, pyrotechnic inflators include a quantity of solid gas generant material which produces the inflation gas during an energetic reaction (e.g., combustion). Another type of inflator is generally referred to as a hybrid inflator. A hybrid inflator includes a quantity of stored gas under pressure, which is released upon activation by the collision sensor. Releasing stored gas alone, however, results in an initially large pressure output which quickly tapers off, which is not desirable for the inflation of the cushion and protection of the occupant. To avoid this, hybrid inflators also include a quantity of the gas generating pyrotechnic material.
It is well known that heating a gas will increase its pressure. As such, the combustion of the pyrotechnic material in a hybrid inflator will heat the stored gas, increasing its pressure. This permits a smaller quantity of gas to be stored, reducing the size of the pressure vessel holding the stored gas. Additionally, the pyrotechnic material may itself produce gas which will combine with the stored gas to inflate the cushion, further reducing the quantity of stored gas required. A further advantage of using the pyrotechnic material is the possibility of combusting the pyrotechnic material at a delayed time. Specifically, if the pyrotechnic material is fully combusted before the stored gas is released, the pressure of the stored gas will be increased, but the release of this gas will again result in a large initial pressure which quickly tapers off. However, delaying the combustion of the pyrotechnic material until during the release of the stored gas will provide a more sustained pressure, and can even be timed to provide an initially lower pressure which increases smoothly before tapering off gradually. As such, hybrid inflators may provide a very advantageous inflation of the cushion.
These advantages have contributed to the commercial success of a known model of hybrid inflator, similar to that show in FIG. 23 of U.S. Pat. No. 5,230,531 to Hamilton et al., which is included herein by reference. While this inflator has advantages, it also has drawbacks. One of these is the production of particulates by the pyrotechnic material. Specifically, the pyrotechnic material has been a reactive compound (such as potassium perchlorate KClO.sub.4) held in the desired solid shape with the use of a plastic (PVC) binder. During combustion of the pyrotechnic material, this pyrotechnic material forms a high percentage of KCl, which forms very fine particles which are entrained within the gas flow from the inflator. These particles are sufficiently small that it has not been possible to filter them from the gas.
While these particles are not believed to be a health hazard, they are sufficiently fine that they create the appearance of smoke. This obviously has a detrimental psychological impact upon the occupant of a vehicle, who may erroneously believe that the vehicle is on fire following the collision.
One solution to this problem is provided in U.S. Pat. No. 5,240,283 to Kishi, which shows a cushion provided with a main volume inflated directly by the inflator, and one or more secondary volumes. These secondary volumes receive the gas forced from the cushion when it is compressed by the body of the vehicle occupant, rather than venting this gas to the vehicle interior. As such, this arrangement does not eliminate the particulates, but seeks to retain them within the cushion. While this could be a solution, use of a porous cushion material (which is often desirable) will still permit some of the particulate to escape the cushion through the fabric itself. Additionally, it is often desirable to alter the airbag performance by altering the venting characteristics from the cushion. This is made more difficult with the use of reservoirs as in the Kishi patent.
Another solution is shown in U.S. Pat. No. 5,602,361 to Baglini et al. This patent shows the use of a different formulation of pyrotechnic gas generant in a hybrid inflator. While this arrangement provides little or no particulate, it suffers from a different drawback. Specifically, the pyrotechnic generant requires reactive gas, such as oxygen, to be mixed with the stored gas. This reactive gas is required for proper combustion of the pyrotechnic generant. If this inflator suffers from a leak, the stored gas (with the reactive gas mixed therein) will not be present if the inflator is later fired. This will cause incomplete combustion of the new formulation pyrotechnic generant, and result in the production of an undesirable level of toxic gas. As such, this arrangement relies heavily upon the operation of a pressure sensor to cancel activation of the inflator in case of a leak. The possibility of creating toxic gases, should the pressure sensor fail, renders this arrangement less desirable.
Avoiding the production of unfilterable particulate while eliminating the possibility of creating toxic gases would greatly improve existing hybrid inflators, and is the subject of the present invention.